1. Field of the Invention
The present invention relates generally to fiber-optic corporate feeds and more particularly to a corporate feed network for phased array antennas using laser diodes.
2. Description of Related Art
Corporate feed networks for RF/microwave MMW signals are conventionally realized using waveguide or microstrip. Such distribution networks are heavy and bulky, which is undesirable, especially for corporate feed networks for phased array antennas in airborne radar applications. Fiber optic technology affords a significant bulk and weight savings.
Fiber-optic technology using laser diodes is quite desirable in providing corporate networks for phased array antennas, since the hair-like fibers can handle a very wide bandwidth. However, there are drawbacks to the two conventional methods of realizing fiber-optic distribution networks shown in FIGS. 10 and 11. In FIG. 10, the RF power from the exciter is split into many channels by a RF power divider (such as a Wilkinson power divider). Each channel should be individually impedance matched to the laser diode impedance. The drawbacks to this method are that RF power dividers and impedance matching networks are:
(a) power inefficient due to required wide band impedance match and Wilkinson divider; PA1 (b) heavy and bulky; and PA1 (c) bandwidth limiting.
The RF exciting source must have high power to overcome the inefficiencies of the scheme. The bandwidth of known available Wilkinson power dividers is less than that of the proposed invention.
In FIG. 11, the optical signal is divided into many channels as opposed to the scheme of FIG. 10 which divides the RF power. Since the method of FIG. 11 involves the division of the optical signal, a very high power optical source is required. High power solid-state laser diodes with microwave frequency response are not currently available. Also, this system is inefficient for broad band operation where resistive impedance matching to the laser diode is required and because the RF splitting loss to each feed increases in a squared relationship.
Stacked array laser diodes have been constructed of monolithically connected series laser diodes using a multiheterostructure. Such stacked arrays are used for increasing the optical pulsed power by summing the contributions from each device. Conventional stacked arrays can only be operated at very low duty cycles, on the order of less than 1%, because of the thermal problems associated with poor heat sinking to all but the bottom laser diode in the array. The average power generated by such stacked array diodes is thus quite low. Furthermore, the conventional stacked array is not generally useful, nor have they ever been used or considered for use, in corporate feed networks, since the array does not distribute the signal to a plurality of optical fibers connecting to for example, antenna array elements and is generally not matched to the RF impedance of the transmitter circuit.
Hence, despite the advances made in fiber optic technology and laser diode technology, there has heretofore been no means for realizing a corporate feed opto-electronic network for distributing optical energy to microwave devices such as phased array antennas that takes advantage of the wide bandwidth and light weight/volume of fiber-optic components.